Any active domain name that's added in a hosting account includes a number of records that enable it to work properly. In the common case, the name servers (NS records) show where the Internet domain is hosted and they're set on the side of the registrar company, but on the lower level there are certain records created through the hosting company like A (IP address of the website/server), MX (email server), SRV (using a domain address for non-website purposes on a certain port number such as Voice-over-IP), and many others. Having access to these records and by having the ability to edit them, you could have your site with company A and your e-mails with company B, for illustration. This offers you more flexibility because you can select the best provider for every single one of these services and you don't need to stick to one company. Whenever you select a hosting company, you have to make sure that you have access to all DNS records freely as some providers have an additional fee for this feature.